In order to investigate rapid variable stars the multi-channel spectrophotometer MEKASPEK was developed at the Universitäts-Sternwarte München between 1989 and 1993. This project was supported by the Deutsche Forschungsgemeinschaft (DFG-Kz. Ba867/2-1, Ba867/2-2). MEKASPEK allows to measure rapid changes in intensity on very short time scales with high precission and is therefore an especially well-suited instrument for the investigation of rapid variable phenomena as they can be found in dwarf nova eruptions, flare stars, eclipsing binaries and in periodic or chaotic brightness variations (e.g. flickering). High precission of earth-bound photometric measurements requires not only detailed information about the actual atmospheric extinction but also an accurate transformation of the measurements performed within the instrumental system into a well defined photometric standard system. Therefore MEKASPEK has been constructed in a way which allows for a precise determination of the actual extinction parameters as well as an accurate transformation to any broadband photometric standard system. At the same time high time and spectral resolution was achieved. Fig.1 shows the main components of the instrument.
Fig. 1: Main components of the multi-channel spectrophotometer MEKASPEK.
Four fiber bundles, which can be positioned by computer control within the focal plane of the telescope guide the signals of four different sources into a double-prism spectrograph. There the light is dispersed and imaged onto the photocathode of a 2D photon-counting detector (MEPSICRON). Recording of the spectra is done by a VME-Bus computing system. A CCD-camera with integrated imagetube is used to select the different objects and to automatically guide the telescope during the observations.
Fiber Input Channels
Four fiber bundles are used to guide the light of four different sources (selected object, 2 nearby comparison stars and sky background) from the focal plane of the telescope into a double-prism spectrograph. The use of fiber bundles serves for the following purposes:
Fig. 2: Positioning unit with mounted fiber bundle.
The rectangular entrance slit of the double-prism spectrograph is formed by the four fiber bundles. Fig. 3 shows a view into the spectrograph. In order to achieve high efficiency the light of the four sources is split by means of a dicroic filter into a blue and red wavelength path. Both light paths are dispersed separately by a prism which is optimized for the blue and red wavelength region respectively. A similar second dicroic filter is used in combination with the camera optics to image the blue and red spectra close to each other onto the 2D photon-counting detector. This arrangement guarantees a constant high efficiency of more than 65% throughout the whole optical wavelength range between 370 and 990nm at a spectral resolution of dlambda/lambda=50.
Fig. 3: Double-prism spectrograph. The collimator optics is situated at the upper left side. In the middle the two prisms for the red (above) and blue (below) spectral ranges are visible. Next to the prisms (left and right) are the two dicroic filters. Below the left dicroic filter a mirror is attached to the housing of the spectragraph to reflect the light of the blue wavelength path towards its prism. The camera optics is visible at the lower right bottom of the image.
In order to register the spectra of the four sources a 2D photon- counting dectector (MEPSICRON) is used. The spectra are imaged onto the photocathode of the detector (diameter 25mm) where electrons are created via the photoeffect. A cascade of 5 microchannel plates is used to generate an electron avalanche from each electron, which then hits a resistive anode. The center of position of the electron avalanche and therefore of the infalling photon is determined by electronic hardware. Up to 100000 positions per second of detected photons can be determined. In order to diminsh thermal noise the detector is cooled to -20 degree Celsius by means of peltier elements. The positions of the registered photons are stored by a real-time VME-bus computer system. Overall up to 100 spectra per second of the four sources can be measured.
Fig. 4: 2D, photon-counting detector MEPSICRON. The detector with its photocathode (in front) where the spectra of the sources are imaged upon, is to be seen in the middle of the image. On the right the detector housing with including peltier cooling, on the left preamplifier and electronics for determination of the position of the infalling photons are visible.
Photometric intensity measurements from ground are always hampered by atmospheric extinction which causes erratic intensity variations on short time scales. In order to determine the intrinsic variations of the measured object the influence of the earth's atmosphere has to be determined. This is achieved by MEKASPEK through the simultaneous measurement of target object, one or two comparison stars of constant brightness and the neighbouring sky background in combination with a special reduction algorithm applied to these simultaneous measurements.
In a first approximation variations of the atmospheric extinction have the same influence on object and comparison star(s) measurements. Extinction variations also leave typical traces in the sky background data, which allow to distinguish them from possible instrumental effects or intrinsic variation (e.g. moon rise/set).
In the course of the reduction procedure the simultaneous measured sky background is subtracted from the measurements of object and comparison star(s). Afterwards the measured count rates of the object are divided by those of the comparison star. Fig. 5 shows an example for one colour channel: in the upper three panels the original simultaneous measurements of object, comparison star and sky background are displayed. Erratic atmospheric extinction variations strongly influence the recorded data and hide any intrinsic variations of the object. After applying the reduction procedure intrinsic variations of the object show up. The lower panel in Fig. 5 shows the reduced measurements. Atmospheric extinction variations simply cause an increase of noise in the reduced data. Therefore the principle of simultaneous measurements in combination with the special reduction algorithm allows to perform photometric measurements even under widely varying atmospheric conditions.
Fig. 5: Reduction procedure. From top to bottom the simultaneous measurements of object, comparison star and sky background summed up to one colour channel are displayed. In the lower panel of the figure the reduced measuremnts of the object are plotted.
Award for MEKASPEK
In the framework of the special exhibition ``EXEMPLA 1995 - Das Handwerk und die Metropolen'' the Universitäts-Sternwarte München took part in the 47th international handscraft exhibition between march 11th and 19th 1995 in Munich. On an exhibition stand historical as well as modern high-tech instruments of earth-bound astronomy were shown. For both instruments MEKASPEK und MONICA, which were built in the course of two PHD works at the Universitäts-Sternwarte München between 1989 and 1996 the Universitäts-Sternwarte München was awarded the socalled ``Staatspreis der Bayerischen Staatsregierung'' for outstanding contributions at the international handscraft exhibition in Munich.
Observations with MEKASPEK
MEKASPEK has been used in the course of different scientific projects at the 80cm telescope (Fig. 6) at mount Wendelstein situated in the Bavarian Alps, at the different telescopes (1.23m, 2.2m (Fig. 7) and 3.5m) at Calar Alto in southern Spain as well as at the 2.2m telescope of the European Southern Observatory (ESO) at La Silla, Chile.
Fig. 6: MEKASPEK at the 80cm telescope of the Universitäts-Sternwarte München at mount Wendelstein in the Bavarian Alps.
Fig. 7: MEKASPEK within its green carriage, mounted at the 2.2m telescope of the Max-Planck-Institut for Astronomie at Calar Alto in southern Spain.
Cataclysmic Variables - IP Pegasus
In 1990 a long-term observing campaign of the cataclysmic variable IP Pegasus was started at the Universitäts-Sternwarte München. Within 6 years measurements were performed at the 80cm telescope at mount Wendelstein, as well as at the 2.2m and 3.5m telescopes of the Max-Planck-Institute for astronomy at Calar Alto in Spain. The project aimed to study the course of a dwarf nova outburst and to search for typical long-term variations.
Cataclysmic variables are close binary systems with mass exchange, consisting of a white dwarf and a late-type main sequence star. Due to the close gravitational coupling and the compactness of the white dwarf mass is transfered from the main sequence star to the white dwarf. Due to conservation of angular momentum an accretion disk around the white dwarf builds up. At the position where the mass stream hits the disk shocks are generated resulting in a locally heated disk region the socalled hot spot. When the hot spot turns into the observer's view a characteristic increase in brightness is oberserved.
The accreting mass increases the local density and temperature within the disk. When a critical combination of density and temperature is reached the viscosity is increased dramatically. This causes part of the accreted material to be dumped onto the white dwarf thereby releasing part of its potential energy as radiation, mainly in the UV and optical wavelength range. A socalled dwarf nova outburst is observed.
The cataclysmic variable IP Pegasus shows an eclipse of its
accretion disk and its white dwarf every 3.8 hours. This eclipse can
be used to check for variations in the orbital period of the system.
Analysis of the performed long-term observations shows a periodical
change of the orbital period of the system on a time scale of 4.7 years.
Such a period variation can be explained in terms of a dark companion
in the system with a possible mass between
0.08 and 0.16 solar masses. Fig. 8 shows a typical light curve of IP Pegasus
over two orbital cycles (above) and the measured period variations
on a time interval of 6 years.
Fig. 8: Orbital light curve (two cycles) of the cataclysmic variable IP Pegasus (above). Below the periodic variation in the orbital period over a time scale of 6 years as obtained from the long-term measurements can be seen.
Flickering - SS Cygni
The cataclysmic variable SS Cygni was observed with
MEKASPEK at the 80cm telescope
at mount Wendelstein. This observing campaign aimed to investigate erratic
intensity variations on time scales of seconds, the socalled flickering.
By means of the provided spectral resolution by MEKASPEK, it was possible
to perform this investigations not only in a single colour channel, but
in selected wavelength ranges adapted to the scientific aim. The emission
lines of hydrogen as well as the neighbouring continuum was monitored simultaneously
with a time resolution of 2 seconds (see Fig. 9). A comparison of the data
showed that flickering is mostly prominent in the continuum and barely visible
in the lines. Therefore the region close to the white dwarf and the region
of the hot spot could be located as the origin of flickering.
Fig. 9: Simultaneously recorded light curves of the cataclysmic variable SS Cyg in the emission line H-alpha (above) and the neighbouring continuum (below). Obviously the erratic intensity variations are most prominent in the continuum.
In July 1996 the chain of nuclei of comet Shoemaker-Levy 9 hit Jupiter. Because the impacts themselves occured at the backside of Jupiter (as seen from the Earth), no direct earth-bound observations of the impacts were possible. Due to the expected energy release in the range of 10e23 to 10e25 erg/s observers expected to see a reflexion of a single impact on one of the inner (Galilean) moons of Jupiter. Therefore an attempt was made to measure such a "light echoe". During impact L of comet Shoemaker-Levy 9 MEKASPEK was used at the 2.2m telescope of the Max-Planck-Instituts für Astronomie at Calar Alto in southern Spain to obtain high-time resolved spectrophotometry of the Galilean moon Europa.
Fig. 10 shows the light curve of Europa during the impact L with a time resolution of 0.5 seconds in the colour channels B, V and R. The exact time of the impact at Jupiter is marked by an arrow. The data show a short increase in brightness at the time of impact, however, only within the noise. Therefore no secure detection of a light echoe from the impact can be claimed.
Fig. 10: Light curve of Jupiter's moon Europa during impact L of comet Shoemaker-Levy 9 at Jupiter.
In collaboration with the Max-Planck-Institut für extraterrestrische Physik at Garching two flare stars were observed in the x-ray and optical wavelength range. Flarestars show irregular intensity outbursts on short time scales with rising times in the range of seconds and falling times of minutes to hours. Flares are created by variations in the magnetic fields in the outer parts of a star's atmosphere. As a consequence high-energy particles (protons and electrons) are accelerated into the lower parts of the atmosphere where their kinetic energy is released as radiation.
In order to study this process in more detail, in January 1992 a simultaneous observing campaign of multiple flarstars was initiated. X-ray radiation was measured by the x-ray satellite ROSAT, whereas the optical radiation was measured simultaneously with MEKASPEK at the 1.23m telescope of the Max-Planck-Instituts für Astronomie at Calar Alto and at the 80cm telescope at mount Wendelstein. Multiple flares could be measured. A comparison of the optical light curve with the x-ray data showed that in every case the optical flare precludes the x-ray flare. Time delays of 1 to 4 minutes were measured. Fig. 11 shows optical and simultaneous x-ray data of the flarestars EV Lacerta and EQ Pegasus. Clearly the time delay is visible.
Fig. 11: Light curves in the optical wavelength range and x-ray light of the flarestars EV Lacerta and EQ Pegasus, obtained with ROSAT and MEKASPEK.
The following publications are related to the MEKASPEK project: